An electric discharge machining device is a device that processes a workpiece by generating arc discharge between a processing electrode and the workpiece. The electric discharge machining device requires a power supply (discharge processing power supply) for generating the arc discharge, and various structures have been suggested for the processing power supply.
Examples include an electric discharge power supply that is provided with a first power supply unit for generating a voltage pulse to make the processing electrode (hereinafter, “electrode”) positive with respect to a workpiece and a second power supply unit for generating a voltage pulse to make the electrode negative with respect to the workpiece, and alternately drives these two power supply units to apply positive and negative discharge pulses to space between the electrode and the workpiece (for example, see Patent Document 1, FIG. 35).
The structures of the power supply units are briefly explained. The first power supply unit includes a capacitor Cq that accumulates electrical charge therein, a DC power source Vs that recharges the capacitor Cq, a charge resistor Rs arranged on the charging path, a switching element Su that feeds the electrical charge accumulated in the capacitor Cq into the space and thereby generates a pulse of discharge voltage, a backflow prevention diode D2 connected in series to the switching element Su, a switching element S1 connected in parallel to the space to short this space, and a backflow prevention diode D1 connected in series to the switching element S1. The second power supply unit includes a capacitor Cr that accumulates electrical charge therein, a DC power source Vt that recharges the capacitor Cr, a charge resistor Rr arranged on the charging path, a switching element Sv that feeds the charge accumulated in the capacitor Cr to the space and thereby generates a pulse of discharge voltage, a backflow prevention diode D4 connected in series to the switching element Sv, a switching element St connected in parallel to the space to short this space, and a backflow prevention diode D3 connected in series to the switching element St.
Furthermore, the discharge machining power supply device described in Patent Document 1 executes the following operations. First, the capacitor Cq is charged by the power supply Vs of the first power supply unit by way of the charge resistor Rs, and the energy charged in the capacitor Cq is supplied to the space between the workpiece and the electrode (hereinafter, “the inter-electrode space”, considering the workpiece as an electrode), when the switching element Su is turned on. Here, even if the voltage generated between the electrode and the workpiece (hereinafter, “inter-electrode voltage”) increases and leads to discharging, only a limited amount of energy is supplied. Furthermore, the voltage charged in the capacitor Cr by the power supply Vt (reverse voltage with respect to the voltage charged in the capacitor Cq) is applied to the inter-electrode space so that the discharge current quickly disappears. In other words, in the electric discharge machining device described in Patent Document 1, the first power supply unit applies a voltage to perform discharge processing, and then the second power supply unit applies a voltage to prevent discharging from recurring. Thus, a current that tends to flow when discharge processing is not taking place can be blocked, which improves the surface roughness.
In the above control, when discharging does not occur, the charge remains between the electrodes. The inter-electrode state therefore needs to be reset before the next voltage pulse is applied. For this reason, control has to be performed by turning the switching element S1 and the switching element Sv on at the same timing to reset the inter-electrode state even when there is no discharging between the electrodes.
In other words, this discharge machining power supply device performs control to develop a voltage shifted from the capacitor Cq (Cr) between the electrodes in each voltage pulse application cycle, regardless of whether there is discharging, and to reset the shifted voltage in each cycle. The device also performs control after the inter-electrode voltage is reset to raise back the inter-electrode voltage by turning the switching element Su (St) on and applying the voltage developed from the capacitor Cq (Cr).    Patent Document 1: Japanese Patent Application Laid-open No. 2003-205426